The present invention relates to an exhaust system, and, more particularly, to a catalytic exhaust system for an internal combustion engine in which a catalyst is preheated by a fuel-operated burner into which fuel is fed from a fuel tank by way of a pump and a fuel pipe.
A burner described in DE-OS 41 34 449 is used for bringing the catalyst to its operating temperature more rapidly and comprises essentially a combustion chamber and a shell housing which surrounds the chamber by means of an annulus into which air is pressed by way of an air feeding pipe. Furthermore, the fuel is fed into the combustion chamber via a fuel pipe and, in the chamber, is absorbed by a fiber glass mat and finally burnt by an electrical ignition device provided on the burner.
Although a burner of this type can be operated by any combustible liquid, as well as by gas, normally the fuel present in the vehicle is used for this purpose which is fed into the combustion chamber by way of an already present vehicle pump at approximately 3 to 4 bar. Since this pressure is relatively low, a relatively poor combustion is caused in the combustion chamber.
In addition, also in the case of a comparatively low delivery pressure of the fuel, the self-emission of the burner is relatively high, particularly at the time of the start of the internal-combustion engine. Naturally, these disadvantages could be avoided by a very high-performance electric pump assigned specifically to the burner. However, this pump is not only relatively heavy and expensive but also requires an additional pressure regulator and an electrical connection. Finally, the electric pump may also generate unpleasant noises.
It is therefore an object of the present invention to provide, in an exhaust system of the aforementioned type, a device by way of which the fuel is fed to the burner with a pressure which ensures an optimal atomization and combustion while doing without an additional pump assigned specifically to the burner.
For achieving this object, the present invention provides an increased pressure in the fuel pipe via an output amplifier. By means of the output amplifier provided in the fuel pipe according to the invention, the fuel pressure existing in this fuel pipe, which normally amounts to approximately 3 bar, can be increased considerably without any additional pump, specifically to approximately 10 bar.
An output amplifier in accordance with the present invention is lower in cost, lighter, sturdier with respect to the operation and considerably quieter than an electric pump which is assigned specifically to the burner, and is therefore correspondingly powerful. The output amplifier of the present invention comprises a differential piston unit whose large and small piston can each be displaced longitudinally in a housing. The housing which surrounds the small piston is connected with the burner so that the fuel is introduced into this burner at the desired high pressure of approximately 10 bar.
The large piston of the differential piston unit is provided in a longitudinally displaceable manner in a hollow-cylindrical receiving housing. A guide housing for receiving the small piston is arranged on an end area by way of a ring flange. One currently contemplated embodiment of the present invention is configured such that the guide housing projects axially from the receiving housing containing the large piston in the area of an end wall, and the small piston also is arranged in this guide housing in a longitudinally displaceable manner. A pressure spring, which acts via prestress upon the large or the small piston is supported on the burner-side end wall of the receiving housing or of the guide housing. When the large piston is not acted upon, this pressure spring will displace the differential piston unit into the area of the end wall of the receiving housing which is provided with the fuel pipe. The small piston always engages in the guide housing. The differential piston unit is penetrated by a center passage bore which changes at the free end of the small piston into a return valve which opens, when the differential piston unit is returned, in the direction of the guide housing. This return valve expediently consists of a spring-loaded ball or similar closing element.
Since a leakage may easily form during the operation of the differential piston unit, the receiving housing is advantageously arranged inside a fuel tank, in which case the pressure chamber of the large piston is connected by way of a fuel pipe with a fuel pipe fed by the fuel pump. The receiving housing, the guide housing and/or the differential piston unit may be made of metal, of a plastic material, or of a ceramic material.
After a cold start, an exhaust gas catalyst is preheated by the burner normally only for approximately one minute. The reason is that, subsequently the operating temperature of the exhaust gas catalyst is maintained by the exhaust gas flow. When the engine is switched off shortly after the cold start, however, fuel must again be fed to the burner for another, or possibly repeated (thus, intervals), preheating of the exhaust gas catalyst. This is achieved by providing a flow valve in the large piston of the differential piston unit, or an axially displaceably arranged control ring in the high-pressure chamber of the guide housing which has at least one lateral passage opening. By virtue of the foregoing, the differential piston unit is, immediately after its sliding-in, slid out again by the admission of pressure medium to the interior end face of the large piston and by the spring effect so that fuel can flow again into the high-pressure chamber of the guide housing. According to the constructive development and design of the output amplifier, this operation may be repeated an arbitrary number of times.